1. Field of the Invention
The present invention relates to optical information recording/reproduction apparatuses and methods, in which recording or reproduction is performed using three or more levels of information pits or marks. In particular, the present invention relates to an optical information recording/reproduction apparatus and method which allow averaging of heat interference in recording so as to reduce the effect thereof.
2. Description of the Related Art
Research and development have been promoted with a view to increase recording density. For example, techniques have been developed for increasing the efficiency in multilevel recording/reproduction without changing the size of a light spot.
For example, the present inventor proposes a multilevel recording/reproduction technique in Japanese Patent Application Laid-Open No. 5-128530. In this technique, multilevel information is recorded on an information track of an optical information recording medium, using a combination of the length of an information pit in the track direction and the amount of shift of the information pit in the track direction with respect to a reproduction light spot. Japanese Patent Application Laid-Open No. 5-128530 also describes a reproduction method for reproducing the recorded multilevel information pit using correlation between a detection signal which has been learned and a detection signal which is obtained from the light spot.
In addition, in “ISOM 2003, Write-once Disks for Multi-level Optical Recording, Draft Collection, Fr-Po-04”, published by ISOM, which is an international symposium in the field of optical disc research, an experiment report is presented.
In this report, eight-level recording/reproduction is performed, using an optical system with a violet light source of a 405 nm wavelength and an NA of 0.65, on an optical disc with a track pitch of 0.46 μm provided with a virtual region for recording one information pit (hereinafter referred to as a cell), whose width in the track direction is set to be 0.26 μm.
Setting of the eight-level information pits is performed, for example, by dividing the length of a cell in the track direction shown in FIG. 10 into sixteen (16-channel bits) and setting the level of an information pit as follows: level 0: no information pit to be recorded; level 1: having a width of 2 channel bits; level 2: having a width of 4 channel bits; level 3: having a width of 6 channel bits; level 4: having a width of 8 channel bits; level 5: having a width of 10 channel bits; level 6: having a width of 12 channel bits; and level 7: having a width of 14 channel bits.
Japanese Patent Application Laid-Open No. 2002-245626 describes another multilevel recording/reproduction technique. FIG. 11A is a schematic diagram illustrating a case where information pits 15 are randomly recorded on a track 14 provided on the optical disc. This figure also illustrates a relationship between the information pits 15 recorded on the track 14 and a light spot 16. For example, when a phase change material is used as an erasable recording material in this technique, the light spot 16 is irradiated onto the optical disc, and the light intensity and timing of each of a recording pulse, an erasing pulse, and a cooling pulse are adjusted. This causes the shape of the information pits 15 to be changed, resulting in the formation of information pits with a plurality of levels. In FIG. 11A, for convenience, these information pits 15 are shown as rectangular information pits.
FIG. 11B illustrates a drive waveform of a semiconductor laser, i.e., a recording strategy, which is associated with the recording of the information illustrated in FIG. 11A. In the figure, “Pw” denotes a recording power, “Pe” denotes an erasing power, and “Pc” denotes a cooling power. In this strategy, as shown in FIG. 11B, while the pulse width of the recording power Pw is set constant, the pulse width of the cooling power Pc is changed in accordance with the sizes of the information pits to be recorded.
There are other recording strategies, for example, in which the pulse width of the recording power is relatively changed, or the level of each of the powers is changed. In any of these strategies, no recording power is irradiated for information having a level (level 0 in FIG. 11A) for which no information pit is recorded, as shown in FIGS. 11A and 11B.
When information is reproduced, recorded information pits are scanned using a light spot, and reflected light from the information pits is received by a photodetector. Then, sampling is performed at a timing when the center of the light spot is irradiated on the center of a cell with respect to the track direction. This results in a distribution of amplitudes of a reproduction signal which correspond to the individual levels, as shown in FIG. 12.
As a standardized configuration, the amplitude level of a reproduction signal obtained when there are consecutive information pits of level 0, i.e., there are no written information pits, is set as “1”, and the amplitude level of a reproduction signal obtained when information pits of level 7 are consecutively recorded is set as “0”.
The value of a reproduction signal (amplitude) level corresponding to each of the information pit levels has a range, since a target information pit is affected by information pits written in the preceding and succeeding cells (inter-symbol interference). As shown in FIG. 12, when the amplitude distribution of a reproduction signal at a certain level is overlapped with that of the reproduction signal at an adjacent level, a fixed threshold value cannot be used for discriminative detection of signal levels.
In general, the effect of such inter-symbol interference is canceled by performing processing on the reproduction signal, such as waveform equalization using a coefficient obtained from the transfer function of an optical system. This separates amplitude distributions corresponding to information pit levels, allowing detection of signal levels using a predetermined threshold value.
When further higher recording density, for example, more than 30 Gbit/inch2, is attempted using an optical system with a violet light source of 405 nm wavelength and an NA of 0.85, the width of a cell may have to be 200 nm or less when eight-level information pits are applied.
In addition to the inter-symbol interference, information pits in adjacent cells can be affected by heat created during recording, i.e., heat interference. Through this heat interference, the size of an information pit is increased or decreased in accordance with the sizes of information pits written in the preceding and succeeding cells. The possibility that the heat interference occurs increases with a decreasing cell width. In addition, the heat interference causes the center position of the information pit to be deviated forward or backward in the track direction in accordance with the sizes of the information pits written in the preceding and succeeding cells, resulting in an error in the reproduction signal.
The heat interference is caused by a difference between information pits in terms of the magnitude of power or irradiation time of the light irradiated in recording of the information pits. Thus, a heat difference is maximized between information of level 7 for which the largest information pit is recorded, and the information of level 0, for which no information pit is recorded. Particularly, in the known technique described with reference to FIG. 11B, the information of the level 0, for which no light is irradiated at the recording power, advantageously brings about a significant temperature difference compared with the other levels.